Conventionally, there exists a power recovery system which includes a Stirling engine and a vaporization device (see Patent Document 1 for example).
This power recovery system is constructed by combining the Stirling engine and the vaporization device and is designed for power recovery accompanying a liquid vaporization process.
The Stirling engine has a hot heat exchanging portion and a cold heat exchanging portion. The Stirling engine generates power by supply of hot heat to the hot heat exchanging portion and supply of cold heat to the cold heat exchanging portion.
For example, such a Stirling engine has a cylinder containing a piston. This Stirling engine generates power by actuating the piston by supply of cold heat to one end portion (cold heat exchanging portion) of the cylinder and supply of hot heat to the other end portion (hot heat exchanging portion) of the cylinder.
The vaporization device stores a low-temperature liquid such as LNG therein and vaporizes the liquid by depriving the liquid of its cold heat (latent heat), i.e., by warming the liquid.
Specifically, the vaporization device has a liquid storage portion which stores the liquid therein in such a manner that the liquid is kept in contact with one end portion of the cylinder. By supplying the cold heat (latent heat) of the liquid to the cylinder, the Stirling engine generates power, which is then recovered. In the process of the power recovery, on the other hand, the vaporization device deprives the liquid of its cold heat to vaporize the liquid into gas, which is then recovered.
With such a power recovery system, when heat from outside (hereinafter will be referred to as external heat) is transferred to the liquid storage portion, the amount of cold heat of the liquid in the liquid storage portion to be supplied to the Stirling engine is reduced by the amount of cold heat deprived of by the external heat. This means a reduction in the amount of cold heat to be used for power generation, which leads to a decrease in the power recovery rate of the system. Therefore, the transfer of the external heat to the liquid storage portion is not preferable.
In view of this, a method is conceivable which provides heat insulation for the liquid storage portion by provision of a heat insulating material, such as urethane, around the liquid storage portion. With this method, it is possible to prevent the loss of cold heat of the liquid by reducing the amount of heat to be transferred from outside to the liquid storage portion.
The heat insulating method described above can be expected to bring about a heat insulating effect to a certain degree. However, when the temperature of the heat insulating material is raised by the external heat, the heat insulating effect by the heat insulating material is lowered. For this reason, there is a problem that the power recovery rate decreases with lapse of time.
Patent Document 1: Japanese Patent Application Laid-open No. H11-22550